I think you should check with the organization and venue what photo gear is allowed in the first place? Maybe a big SLR with a pro-looking lens is off limits (check on tickets or website). If so, then don't take that because it may get you in trouble.

Thanks Roel. The info from their website seems fairly clear:

"Items that are restricted in London
2012 venues are:

......
––Large photographic and broadcast
equipment over 30cm in length,
including tripods and monopods.
You cannot use photographic
or broadcast equipment for
commercial purposes unless you
hold media accreditation"

.....

So in theory any of the stuff I mention could get through. I'm not sure if I'm ready to risk the 50-200 - it's by far the most valuable lens I own and my most recent acquisition - I've not used it much at all yet. The session I'm going to ends at 4pm, so in theory there should be plenty of light around. Sufficient to stop down the 70-300?